RTGs have been powering deep space probes since the beginning of our travels into space. It's really the only technology that will keep a craft powered for many years/decades. Without them you simply can't do these kinds of missions.
And they're not that scary. There's no fission (or fusion) happening here. It's just a radioactive sample that heats a surrounding material by it's natural radioactive decay.
He means there is no sustained chain reaction going on. It's neither critical nor supercritical. I have a chunk of uranium ore on my bookshelf doing it right now.
I got it from a geologist buddy of mine a few years ago, I think he said he found it lying around in Arizona. It's fairly harmless, just looks like a rock really (unless you have a gieger counter).
As far as I know, there are no restrictions on such things. You can pick up pieces of uranium glass and old fiestaware on ebay for a couple of bucks (both about as radioactive as my rock :)
Not all isotopic decay is fission. When an alpha particle is emitted it can be considered fission in the broadest sense (though that's somewhat unusual nomenclature). However, when a beta particle is emitted or absorbed it's not fission.
And they're not that scary. There's no fission (or fusion) happening here. It's just a radioactive sample that heats a surrounding material by it's natural radioactive decay.